Viral or plasmid constructs containing a chloramphenicol acetyltransferase (cat) gene, positioned downstream from an authentic viral or host transcriptional control sequence, should provide a convenient system in which to rapidly and quantitatively evaluate antisense oligonucleotides directed towards specific mRNAs. Phase I work will use a retroviral vector containing the cat gene. Cultured cells will be transfected with this vector, and the effects of antisense oligonucleotides on RNA processing will be monitored by assaying for cat gene expression. Oligonucleotides of various length, containing either normal phosphodiester or methylphosphonate linkages, will be evaluated for antisense activity. The addition of terminal acridine residues to the oligonucleotides will also be evaluated for their effect on antisense activity. Demonstration of the efficacy of this approach will drive a Phase II effort in which a human immunodeficiency virus-directed cat expression system will be constructed for evaluation of antisense probes prior to use in an HIV-infected cell line.